The Miraculous Monarch Migration
Ode by Van Johnson
Photo by Mandy Aguilar
Most of us have seen impressive footage of the Great African Migration in Tanzania and Kenya with vast herds of wildebeest, zebras, and antelope which move over distances of up to 1,000 miles as the seasons fluctuate between rainy and dry. Another of the world's truly great migrations takes place in North America, but it is so big, and the individuals are so small, it is impossible to see large portions of it at once -- the many millions of monarch butterflies every autumn.
About every 4th or 5th generation of monarchs are the migrators, also called the super generation. These may be as far descended as to be the great-great-grandchildren of the monarchs that were last in the Mexico wintering grounds! Unlike birds where the juveniles can follow the adults back and forth between the summer and winter grounds for several years until they learn the way themselves, monarch butterflies do not live long enough for the juveniles to be able to follow the adults. How these 3-inch insects with brains the size of a grain of rice are able to travel up to 3,000 miles to one particular grove of trees in Mexico that they have never seen, their parents have never seen, their grandparents have never seen, and maybe even their great-grandparents have never seen, is truly amazing.
Migrator, AKA super generation, individuals are 30% larger than the individuals of the previous three or four generations. Shorter day lengths, cooler temperatures, and declining food sources trigger the females to lay special eggs that will become the migrators. Monarchs in eastern Canada travel up to 3,000 miles in 30 days, or an average of 100 miles per day! How is this possible since they can only flap their wings fast enough to be able to propel themselves at 10 to 12 miles per hour? They surf the relatively warm "cold" fronts which conveniently move from north to south at that time of year. Not only does it enable them to make the migration in approximately 30 days but it also helps them conserve their fat stores, which they will need to be able to live through the winter in Mexico.
The migrators / super generation are not sexually mature and therefore are in reproductive diapause as they make the journey. There would not be enough time for any eggs laid this late in the season to hatch, become very small caterpillars, grow into larger caterpillars, go into chrysalises, transform into adults and those adults get to Mexico before the cold weather. The monarchs are not so much racing the freeze -- but they are unable to fly when the temperatures are below 60°. So it is critical that they stay ahead of the cold weather as they travel south.
In the Mexican mountain grove they roost in oyamel fir trees for winter warmth, and also cluster in large numbers for the same reason. A nearby stream as well as fog help supply their moisture needs, but for calories they live off the fat in their abdomen during the winter since there are no significant food sources nearby. Monarchs traveling south actually gain weight and increase their fat reserves during the trip!! This is partially due to all the gliding they do on air currents when possible to conserve energy.
Most monarchs live 3 to 5 weeks but the migrators / super generation lives up to 8 months. They leave eastern Canada usually in late August and stay at the wintering grounds in Mexico until about the second week of March. As the days get longer and warmer they mature sexually, become more active, mate, fly north and east, the females lay eggs on milkweed, and that generation dies. The new generation hatches, and repeats the process of flying north or northeast, mating, having offspring and dying. This continues for 3 or 4 generations. So it can take up to 4 generations and all summer to go from the Mexican grove to the farthest reaches of the northern states and southeastern Canada, but it only takes one generation and one month to go from Canada to Mexico! "Super generation" indeed!!
POPULATION CRASH---------------------------------
In 1990 it was estimated that there were 1 billion monarch butterflies in North America. As of the 2023 estimate at the Mexican grove the population has declined to 21 million. Now, 21 million may seem like a lot, but remember it is only 2.1% of a billion. In other words we have lost 98% of the monarch butterflies in North America since 1990!
The primary reason for the devastating monarch population decline is habitat loss. Monarch caterpillars can only eat one food and that food is milkweed. Milkweed coexisted on the North American prairies with the grasses for millennia. When the prairies became cropland the milkweed found homes in between the rows of corn and soybeans. The advent of GMO corn and GMO soybean have wreaked havoc on the milkweed population and therefore the monarch population. GMO corn and GMO soybeans are genetically engineered to be resistant to Round-Up and similar herbicides. The fields are then sprayed with these herbicides which kill everything in the field except the genetically engineered crops, and that of course includes killing all the milkweed.
Another contributing factor to population crash is climate change. Hot summers and drought hurt milkweed and nectar plants, and at the same time there have been unusually cold winters at the Mexican grove causing large die offs.
How you can help the declining population of monarchs, the state insect or state butterfly of 7 states including Texas:
Plant native milkweed for caterpillars
Plant native nectar plants for the adults
Do not use insecticide, including spraying for mosquitoes. Fight mosquitoes in the larval stage by eliminating stagnant water and/or using mosquito dunks which are a 100% people- and pet-safe biological control. Apply mosquito repellent to yourself or your clothes, not to your yard. Do not use chemically based yard companies such as ChemLawn/TruGreen.
Water your native nectar plants as needed so they will be blooming and have nectar for pollinators, especially in the fall as the monarchs are migrating. Native plants will often survive with little or no supplemental irrigation but most will not bloom prolifically and have ample nectar if there is very little soil moisture.
Encourage your local towns and counties to not mow milkweed and to plant patches of nectar plants in parks, etc (but not along roadways where a great many butterflies are killed every year), and to manage mosquitoes in the larval stage vs. air spraying.
Volunteer at butterfly gardens
Help educate others
Note 1:
Milkweed is beloved by aphids. So much so that you will almost always see aphids on milkweed that hasn't been treated with pesticide. Therefore, I never buy milkweed unless it has aphids on it!! That is my guarantee it is pesticide-free and will not kill the monarch caterpillars.
Note 2:
In the Dallas area, the peak of the monarch migration usually passes through in the second week of October, although it begins building before that. Two of the best nectar plants in this area for bringing in large numbers of monarchs during the fall migration are Gregg's mistflower (not "blue" mistflower!! -- make sure it has the highly dissected leaves, not the smooth leaf margins) and frostweed. When frostweed is 3 ft high cut it back to 2 ft. When it gets to 4 ft high cut it back to 3 ft. Then do not cut it back the rest of the season. This will not only create literally four times as many blooms, but it will also have the blooms at approximately eye level in October so you can enjoy the butterflies instead of having the blooms 10 ft above the ground.
Note 3:
For simplicity, this narrative dealt only with the eastern population of monarchs, those east of the Rocky Mountains. There is also a much smaller western population primarily in California that also migrates, but the winter grounds are in Southern California. The Rocky Mountains pretty much keep the two populations from mixing very much.